Friday focal mechanisms

This is a bit of an experiment. People seem to quite appreciate my posts that place significant earthquakes in their tectonic context (e.g. #1, #2, #3). However, I can’t write a detailed post for every single one. So I’m wondering if it might be worth producing a brief weekly summary of any significant earthquakes, showing their focal mechanisms together with a brief tectonic interpretation. Below I do this for all magnitude 6+ earthquakes reported by the USGS in the past seven days. The format is admittedly a bit unpolished right now, but I’m interested if you’d be interested in me making this a regular feature.

Saturday 26 June: M 6.7, Solomon Islands, Depth 35 km

A transpressional focal mechanism, with mostly NE-SW compression. The direction of shortening is consistent with the NE subduction of the Australian plate beneath the Pacific plate under the Solomon Islands. Historical seismicity suggests that this quake occured near the subduction interface.
(USGS page)

Wednesday 30 June: M 6.3, S of Fiji, Depth 536 km

Focal mechanism indicates extension. The great depth of the rupture, and it’s location behind the Tonga Kermadec arc, suggests it occurred far down dip on the subducting Pacific Plate, which is stretching under its own weight as it sinks deeper into the mantle.
(USGS page)

Wednesday 30 June: M 6.2, Oaxaca, Mexico, Depth 20 km

Focal mechanism indicates NE-SW compression. This is consistent with the convergence direction of the subduction zone off the west coast of Mexico (Cocos plate subducting beneath the North American plate), but the rupture is too shallow to be associated with the subduction thrust itself. This earthquake is likely caused by strain transferred across the locked subduction boundary is being accommodated by thrust faulting in the overriding plate.
(USGS page)

Friday 2 July: M 6.3, Vanuatu, SW Pacific, Depth 34 km

Focal mechanism indicates E-W compression. In this region, the Australian plate is subducting to the east beneath the Pacific plate, and the depth of the rupture is consistent with it being a thrust close to the subduction interface.
(USGS page)

Comments (9)

I know this is going back a bit, but it fits the subject and was >6.0: the Northridge Quake in LA in January 1994. I lived in the Valley about 5 miles W of the epicenter at the time, and there were several different things about that quake that caught my attention (apart from the magnitude, that is;), and which have bugged me ever since as I have not seen anybody knowledgeable treat this aspect. In particular, IIRC:
1: It was relatively shallow (7 Km?) but consisted of two distinct slip shocks very close together in time and space (close enough we beneficiaries could not tell).
2: It took place close to the center of a long narrow (about 50m by 5mi wide) valley walled on both sides by sandstone (with intrusions) “mountains” about 3500′ high. Apparently the Santa Susanna side rose about 70cm during it. Everybody had to get their property platt boundaries re-surveyed afterwards.
3: USGS Accelerometers measured more than 2g accelerations on all three axes. I lost a lot of cement roof shakes because my house went up then down fast enough their inertia overcame their bend limits. I myself also went up in the air and landed several feet away (the house moved under me whilst I was airborne), and my computer keyboard hit the opposite wall. There was also enough motion in all directions laterally that every room had its pile of (by now) junk neatly forming a conical pile in its center.
4: the typical damage pattern was house: hole: house: hole, i.e. every other house was fine, but the ones between them got severe damage. I was a between house, my neighbors (on the same underlying earth type) were fine.
I am unsure about the math, but the results look to me very like the slips effectively injected impulse signals into a wave-guide, and the time/space differential coupled with reflections from the valley sides resulted in high amplitude standing wave interference patterns. This is a different failure mode than the ones that usually get the press (liquefaction, poor construction), and is probably unique to the San Fernando Valley.
Aftershocks were fun too. They were audible on approach for some time, via the houses bouncing up and down and their roofs acting as speaker cones as the wavefront passed them.
I’d be interested to see some visualization (or other analysis) of the movements of the valley floor in that quake, and comparisons with more typical quakes not confined in the same way.